The only way I could think of Joker being on trial is if they held Dent becoming Two-Face until the third film, but if not...I don't think we would have seen Joker on trial in the threequel. But, Joker being on trial regardless would be a very interesting idea indeed.

If Ledger were still alive (I think his contract was for multiple films too), I could see that being the way to start of Nolan's third Batman film or somewhere in there.

The Joker being "held accountable" for his crimes against the city. It'd be a completely different story/film too. Who the hell knows.